This invention pertains to an electrode for resistance spot welding wherein the electrode is made of dispersion strengthened metal as well as a process for reorientating the coaxial grain structure of extruded dispersion strengthened metal parts to provide substantially improved products by pressure forming a blank to provide a non-axial grain structure in the product such as a welding electrode.
Dispersion-strengthened metal products, such as copper dispersion strengthened with aluminum oxide, have many commercial and industrial uses wherein high temperature strength properties and high electrical and/or thermal conductivities are desired or required in the finished product. Such commercial uses include many electrical uses such as for example, contact points, switches and switch gears, transistor assemblies, wires for solderless connections, wires for electrical motors, and many other uses requiring good electrical and thermal conductivities together with good strength and hardness at elevated temperatures.
Dispersion strengthening has been recognized in the past as a method for increasing strength and hardness of metals. A solid solution alloy comprising a relatively noble matrix metal having relatively low heat or free energy of oxide formation and a solute metal having relatively high negative heat or free energy of oxide formation can be dispersion strengthened by heating the alloy under oxidizing conditions to preferentially oxidize the solute metal. This technique of oxidizing the solute metal to a solute metal oxide is known in the art as in situ internal oxidation or more simply internal oxidation.
Several processes for internal oxidation have been suggested, such as disclosed in the Schreiner patent, U.S. Pat. No. 3,488,185; the McDonald patent, U.S. Pat. No. 3,552,954; the Grant patent, U.S. Pat. No. 3,179,515; as well as commonly assigned U.S. Pat. No. 3,779,714 and incorporated herein by reference. U.S. Pat. No. 3,779,714 provides an improved alloy-oxidant mixture wherein the oxidant includes an oxide which releases oxygen to oxidize the solute metal of the alloy. The oxidant further includes a hard refractory oxide which is particularly adapted to assimilate the oxidant residue into the dispersion-strengthened metal and becomes dispersion strengthened during thermal coalescence by the hard, refractory metal oxide. The oxidant residue formed during internal oxidation is not required to be removed from the dispersion-strengthened metal but rather is dispersion strengthened by the hard, refractory metal oxide during coalescence to form an integral part of the dispersion-strengthened metal stock. A further improvement is set forth in allowed copending and commonly assigned application identified as Ser. No. 384,028 filed July 20, 1973 which provides improved dispersion-strengthened metals produced by internal oxidation by first recrystallizing the alloy powder prior to internal oxidation to increase the grain size of the alloy to a grain size at least as large as Grain Size No. 6 as measured by ASTM Test No. E-112.
Dispersion strengthened metals are often referred to as super alloys wherein hard stock is produced by subjecting the dispersion strengthened metal powders to consolidation under heat and pressure. The most common method of consolidation is extrusion at temperatures at least above about 1000.degree. F., usually above 1400.degree. F. and preferably about 1700.degree. F. for dispersion strengthened copper. Extrusion for example, can be by a ram-type extrusion press wherein the extrudate emerges in cylindrical or other desired bar stock. The cylindrical extrudate is then often cold drawn to further reduce the cross-section of the rod. Extrusion, however, produces an anisotropic axial grain structure in the direction of extrusion which tends to produce lower hot strength in the transverse direction although maintaining good Strength in the axial direction. The lower hot strength in the transverse direction has been found to contribute to premature mechanical failure of a resistance welding electrode wherein the electrode tip tends to mushroom and crack from repetitive use of the electrode.
Accordingly, it now has been found that dispersion strengthened metal electrodes can be substantially improved by reorientating and/or destroying the anisotropic grain structure in the tip portion of the electrode by causing the tip portion to flow under pressure and produce a localized, fine substantially equiaxed or isotropic grain structure in the electrode tip. The equiaxed or isotropic grain structure in the electrode tip overcomes the deficiencies inherently formed during the consolidating extrusion step and drawing step which produce an axial anisotropic fibrous grain structure.
A primary object of this invention is to reorientate and/or destroy the fibrous coaxial grain structure in extruded dispersion strengthened metal stock by applying axial pressure to an electrode blank to cause flow transverse to the axis whereby an equiaxed grain structure is formed in the electrode tip.
A further object is to provide a process for increasing the overall strength in load bearing portions of workpieces such as welding electrodes by pressure forming and causing flow transverse to the coaxial fiber structure to destroy and/or reorientate the grain structure into a non-axial grain structure such as isotropic or equiaxed grain structures.
These and other advantages will become more apparent by referring to the drawings and the detailed description of the invention.